I. Field of the Invention
This invention relates to a rotor-shaft assembly, and more particularly to such assembly suitable for use in a gas turbine.
II. Description of the Prior Art
Presently, gas turbines are widely used as one of heat engines in various field. As a means for improving the efficiency of the gas turbine, it has been attempted to use ceramics having a high heat resistance and a high mechanical strength for making a turbine rotor. For example, a turbine rotor made of sintered silicon nilride has been developed recently and come to be used in gas turbines for an automobile and for other fields of industries.
A turbine rotor is joined to a shaft and rotated at a high speed within a turbine. A ceramic turbine rotor is intended to enable a turbine to operate at high temperatures. Generally, the circumferential edge portion of the ceramic turbine rotor is exposed to high temperatures which an ordinary heat-resistant metal is incapable of withstanding, for example, to a temperature of 1,200.degree. C. However, the central portion of the rotor is not so heated, rendering it possible to join the ceramic turbine rotor directly to a metal shaft.
An interesting technique of joining a ceramic turbine rotor to a metal shaft, which is under development, is reported on page 24 of a book "Ceramics for High Performance Applications" published in 1974. Specifically, an attempt to join these two members by a so-called "curvic coupling" is reported in the book. The curvic coupling is a face spline and necessitates a particular machining of high precision.
It is not desirable to apply machining of this kind to a ceramic rotor, because the machining of ceramics is very difficult compared with the machining of metal. In addition, if a machining error has taken place, the stress is concentrated onto the erroneously machined portion during the rotation of the rotor, leading to breakage of the rotor. Further, the machining error makes the rotor itself nonusable in some cases, resulting in a low yield of the rotor.